为什么我的索引里面_score是为null呢？

如下是我存储的值。

 "_index" : "supercoach",

      "_type" : "supercoach_wxcard",

      "_id" : "66794",

      "_score" : null,

      "_source" : {

        "addinapp" : 0,

        "avggradtime" : 0,

        "basicWeightScore" : 130,

        "bgimg" : "/common/img/images/bg10.jpg",

        "cityid" : 1,

        "classInfo" : [ {

          "cartype" : "C1",

          "classname" : "贵宾班",

          "createtime" : 1463126879703,

          "id" : 41875,

          "mobile" : "18510649985",

          "openid" : "",

          "price" : 2000,

          "remark" : "周一至周五,两人一车"

        }, {

          "cartype" : "B1",

          "classname" : "普通班",

          "createtime" : 1463126879703,

          "id" : 41874,

          "mobile" : "18510649985",

          "openid" : "",

          "price" : 5800,

          "remark" : "周六至周日,两人一车"

        } ],

为什么_score是为null呢？
我该如何为_score赋值？ 就是每次都会有一个基本值。然后根据不同的数据来变化_score。这个score能自动变吗？我能自己设置么？并且能够根据这个_score来排序